Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble in the chamber, which expels ink through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
Many inks that are described for use in ink-jet printing are usually associated with non-thermal ink-jet printing. An example of such non-thermal ink-jet printing is piezoelectric ink-jet printing, which employs a piezoelectric element to expel droplets of ink onto the print medium. Inks suitably employed in such non-thermal applications often cannot be used in thermal ink-jet printing, due to the effects of heating on the ink composition.
Two inks made using water-soluble dyes, when printed next to each other, tend to bleed and also give reduced waterfastness. Bleed, as used herein, refers to the mutual dye diffusion that takes place when one ink dot is placed next to another on the print medium. If the two dots contain dyes of different hues, then the phenomenon is called color bleed, and is highly undesirable in ink-jet printing.
Bleed control has been accomplished in different ways: (a) printer techniques, where dots are separated, or placed with time lags in between printing adjacent dots; (b) use of fast drying inks where time for ink diffusion is minimal; and (c) use of heater and/or blowers to accelerate drying are examples of some of them.
The use of printer techniques requires complex algorithms in printer control. They slow down the printer throughput. Fast drying inks often do not give satisfactory bleed control due to their tendency to spread on the print media. Heated platens add cost and complexity to the printer.
Microemulsions, which may be defined as thermodynamically stable isotropic "solutions" of water, oil, surfactant, and cosurfactant, have been used to solubilize water-insoluble dyes for ink-jet printing in the past. The function of water is to provide a continuous phase for the microemulsion droplets and it facilitates the formation of microemulsion droplets by entropic means. The oil is a water-insoluble substance which resides primarily in the microemulsion droplets--the discontinuous phase. The surfactant is an amphipathic, surface active, self-aggregating species which is primarily responsible for the formation of microemulsion droplets. The cosurfactant is also an amphipathic species which significantly concentrates in the microemulsion droplets and it affords stability to the droplets.
Cosolvent, a term commonly used in ink-jet technology, refers to a water-miscible solvent having a vapor pressure that is considerably lower than that of water. Cosolvents are usually added to prevent nozzle clogging. Certain cosolvents, however, may also stabilize microemulsion droplets.
The success of microemulsion-based inks has been limited due to the extent of threading or feathering in the resulting print sample. Apparently, the combination of high organic solvent and surfactant concentrations used in these inks causes extensive wetting of the paper fibers and fillers, resulting in feathering; poor edge acuity is the end result.
Attempts have been made to overcome such print quality deficiencies by the use of microemulsion-based inks that are solids at ambient temperatures, but are liquids at elevated temperatures (e.g., 70.degree. C.); see, e.g., U.S. Pat. No. 5,047,084. These inks, however, place additional demands on the printhead and the printer, such as pre-heaters to keep the ink in liquid form prior to firing, and rollers to flatten the solid ink droplets (lenslets) that are formed on the print medium, thus making the product more complex and costly.
A related patent application, Ser. No. 07/853,471, filed Mar. 18, 1992, and assigned to the same assignee as the present application, discloses and claims microemulsion-based inks which incorporate water-insoluble dyes. These inks have true waterfastness, are non-threading, and are bleed-alleviated. The ink-jet inks have a formula comprising: (a) about 0.05 to 0.75 wt % of a high molecular weight colloid; (b) about 0.1 to 40 wt % of at least two surfactants, comprising at least one surfactant and at least one cosurfactant; (c) about 0.5 to 20 wt % of at least one cosolvent; (d) about 0.1 to 5 wt % of at least one water-insoluble dye; (e) about 0.1 to 20 wt % of an oil; and (f) the balance water. These inks form a stable microemulsion, which results in bleed alleviation and excellent line definition.
Commercially available water-insoluble (oil-soluble) color dyes in general do not give the color gamut necessary for ink-jet printing. In part, this is the reason for the current popularity of water-soluble color dyes in ink-jet printing. Such color gamut concerns do not apply to the black dyes. Therefore, an ink set comprising a water-insoluble black dye and water-soluble color dyes would be an attractive approach, if black-to-color bleed were alleviated.